U.S. Pat. No. 3,314,108 discloses an extruder comprising two conical stators and a rotor placed rotatably between the stators. This apparatus comprises supply conduits along which a material to be extruded can be supplied in such a way that the material is fed along one supply conduit into the exterior of the rotor and along the other supply conduit into the interior of the rotor via openings provided therein. The apparatus enables the manufacture of, for example, two-layer plastic pipes, but the layer thicknesses of the pipe or the temperature of the material to be extruded cannot be regulated or changed in any way.
EP 0 422 042 discloses an extruder comprising at least three conical stators and at least two conical rotors placed between the stators. The material to be extruded is fed between the rotor and the stator via supply conduits. The rotors also comprise openings via which the material to be supplied can also enter the rotor and flow to the space between the rotor and the stator provided inside it. The apparatus provides for example multilayer pipes, but it is not possible to regulate the properties of the different layers with respect to each other. Also, the temperature of the materials on different sides of the rotor is the same on each side and there is no possibility of cooling from the outside. Further, regulation of the temperature of the material to be extruded in the extruder must be carried out with difficult separate heating arrangements.
DE 1 454 762 discloses an extruder comprising two feeding devices with an adjustable feed rate. The feeding devices supply the different components of the mass to be extruded to the space around the screw in such a way that the mass components are mixed at the beginning of the screw, whereupon the extruder produces a mass that is a mixture of different components. The screw is used to extrude a one-layer product, whereupon the mixing ratio of the mass components supplied from the different feeding devices is regulated by adjusting the feeding devices. Therefore the apparatus can only provide a mixed one-layer product.
CH 392 045 discloses an apparatus comprising a feeding device which can be used to adjust the amount of the mass flow to be supplied. Also in this case, the mass flow is supplied to the space surrounding the screw, by means of which a one-layer product is extruded, i.e. the apparatus cannot produce products where the properties of the different layers are adjusted separately.
EP 0 153 715 discloses an extruder where material to be extruded is supplied from two hoppers to the outer surface of a tubular rotor. The tubular rotor is provided with grooves such that the materials arriving from different hoppers are first passed to different directions at the outer surface of the rotor and thereafter via the ends of the rotor to the interior thereof. The apparatus is further provided with force-feed rings by means of which the speed of travel of the material at the surface of the rotor can be regulated. The regulation requires a complicated and cumbersome construction in order to be successful, since the force-feed rings require two large rotating drive motors that must rotate against the direction of rotation of the rotor. Further, the feed rings themselves grind the material. The supply arrangement does not in any way ensure the flow of a sufficient amount of the material forward, either, but for example if the supply conduit becomes clogged, it is not possible to supply a sufficient amount of material forward by means of the feed rings even with high speeds of rotation. Further, in the apparatus the regulation of the temperature of the material must be carried out, if desired, with difficult external heating or cooling devices.